In-mold expansion-molded articles prepared by using pre-expanded polypropylene resin particles are characteristic in their freedom in designing the shape, lightness and heat insulation efficiency, i.e., advantages of in-mold expansion-molded articles. When compared with similar in-mold expansion-molded article, they are superior in chemical resistance, heat resistance and distortion recovery rate after compression, compared with in-mold expansion-molded articles prepared by using pre-expanded polystyrene resin particles and also superior in dimensional accuracy, heat resistance and compression strength, compared with in-mold expansion-molded articles prepared by using pre-expanded polyethylene resin particles. Due to these characteristics, the in-mold expansion-molded articles prepared by using pre-expanded polypropylene resin particles have been used as automobile interior parts, core materials for automobile bumpers, and also in various applications such as heat insulators and buffering packaging materials.
Polypropylene resin particles are prepared, for example, by strand-cutting method or underwater cutting method. It is possible to produce uniform resin particles in cylindrical shape with a relatively inexpensive equipment by the strand-cutting method, but production of fine polypropylene resin particles of 2 mg/particle or less in weight by the method resulted in frequent miscutting and breakage of the strands, leading to deterioration in productivity. In addition, there was a problem of relatively low productivity and others, because there is a limit in increasing the number of strands.
It is known that it is easy to initiate granulation and possible to produce uniform-sized polypropylene resin particles at high productivity with less miscutting by the underwater cutting method, because the strand needs not be withdrawn.
Patent Document 1 describes a problem in the method of producing polypropylene resin particles for pre-expanded polypropylene resin particles by the underwater cutting method that the fusion rate of the pre-expanded polypropylene resin particles during in-mold expansion molding is not favorable, and discloses a method of extruding a resin into high-temperature circulating water and cutting the extruded resin in the water, as a countermeasure. However, it is needed to add slightly water-soluble inorganic materials, surfactants and antifoams to the circulating water for prevention of agglomeration, and it is necessary to add the ingredients above consistently for preservation of the concentrations of the additives in the circulating water, which may lead to water pollution.
Patent Document 2 discloses pre-expanded particles prepared from a resin mixture containing 55 to 95 wt % α-olefin-propylene copolymer having a flexural rigidity of 7000 to 14000 kg/cm2 and 5 to 45 wt % linear polyethylene resin having a density of 0.88 to 0.94 g/cm3 as base resin, for the purpose of providing an in-mold expansion-molded polyolefin resin article superior in buffering property, impact resistance and heat resistance. However, the Patent Document does not describe anything about the problem that the fusion rate of the pre-expanded particles prepared by the underwater cutting method is unfavorable.
Patent Document 3 discloses that expanded resin particles having a core layer of a crystalline thermoplastic resin in the expanded state and a sheath layer of an ethylenic polymer having a melting point lower than the thermoplastic resin in the non-expanded state have favorable fusion rate even at low heating steam pressure. However, the method demands a large-scale production equipment, because two extruders are needed.    Patent Document 1: JP-A No. 10-119037    Patent Document 2: JP-A No. 4-253741    Patent Document 3: JP-A No. 10-77359